I am playing through Black 2 right now and am going to give advice on the 3 Pokemon rivaling my level 63 Mienshao for MVP, bare in mind the level of these three is 42, yes, that's right 3 Pokemon are rivaling my MVP for that status despite being 21 levels lower then him.
Zebstrika Top Tier
Type : It's typing is perfect for where you get it because the next gym is the flying one, it also tears Marlon to shreds with STAB Discharge and Wild Charge but I left the zebra Pokemon out of that battle this time round.
STATS : Massive speed and decent offences meant I could solo Skyla at level 33 which is no easy feat even for an electric type..
Movepool : It gets two of it's best moves from level up in the form of Discharge and Wild Charge to smash stuff with before getting Thunder(bolt) from TM as well as Signal Beam from the Driftveil Move Tutor.
Major Battles : okay, there is gonna be some theoymonning here because I've never used a Zebstrika, or any of the Pokemon currently in my team against the E4, so E4 people will be theorymonned
Skyla : Soloed
Drayden : You can do damage but his Pokemon resist electric so it is not a good idea to use him but you could get some revenge kills with Wild Charge
Marlon : Dischage + Wild Charge = GG
Marshel : Same as Drayden minus the resist part I guess, plus Mienshao might outspeed you if you aren't a high enough level
Caitlin : Be careful, her Pokemon can hit hard but Sigliph goes down fairly easily due to its flying typing. If you have Signal Beam it will help alot.
Grimsly : Same as Caitlin but minus the Sigliph bit, also I don't think you can outright K.O Krookodile in one shot, so it should only revenge kill it if you are going to use it against the crocodile, leading with it and using an X Special on him when Liepard uses Fake Out might allow you to OHKO it though.
Shauntel : Drifblim can be taken out with Wild Charge but it can't really take on anything else, Golurk murders it.
Iris : It could possibly take on Archeops and Lapras, but overall should only be used as revive/heal fodder
Lucario : Lucario will be Lucario, Top Tier.
Leavanny Top Tier
Type : Bug/Grass is good because there aren't alot of pokemon in this game that can hit it super effectively
Moves : Gets it's 3 best moves in the form of Swords Dance, X-scissor and Leaf Blade by leveling up and can use Seed Bomb for it's grass STAB until it gets Leaf Blade if it doesn't know it already.
Evolving : You can just grind against that idiot with the Drilbur and Marill just north of Caseltia City once it becomes a Sewaddle, Just take on Sandile on route 4 until then
Stats : Being fairly bulky and easily capable of outpowering Cobalion if you get a good nature is all Leavanny needs, but it can be better if you give it some team support such as pokemon that resist fire, flying and poison.
Major Battles :
Burgh : Leavanny can do well here if it outspeeds Burgh's Leavanny because it can hit with a super-effective Bug Bite
Elesa : Do not take on Emolga or Zebstrika, can do okay against Flaffy though, and even though I said not to it can effectivley revenge zebstrika if it hasn't got to many flame charge boosts and is lower leveled.
Clay : Seed Bomb or Leaf Blade absolutely wrecks everything here not called Excadrill, though it is the same situation as Zebstrika for it.
Skyla : Turned into bird food here plus you should be soloing with a Zebstrika anyhow.
Drayden : It's okay here, can't use it's best move due to resistince though, meaning it should be kept out apart for being fodder.
Marlon : Walk in that gym, Solo Marlon, Walk out with the badge. (Also possible with Zebstrika)
Shauntel : Golurk is the only Pokemon you can safeley kill, and even taking it on is risky
Grimsley : Give Leavanny an X Attack as Leipard Fake Out's and Sweep, but bisharp is a big threat to this strategy because it is a steel type.
Caitlin : Siglyph is dangerous, everything else gets hit hard by X-scissor
Marlon : Resists fighting, gets smashed by rock-type coverage
Iris : Could take out a weakened Lapras, Druddigon or Aggron, should be used as a sacrifice for healing or reviving once they go down.